<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML3.2 EN">
<HTML>
<HEAD>
<META NAME="GENERATOR" CONTENT="DOCTEXT">
<TITLE>MPI_Allgather</TITLE>
</HEAD>
<BODY BGCOLOR="FFFFFF">
<H1 id="MPI_Allgather">MPI_Allgather</H1>
Gathers data from all tasks and distribute the combined data to all tasks 
<H2>Synopsis</H2>
<PRE>
</PRE>
<PRE>
int MPI_Allgather(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
                  void *recvbuf, int recvcount, MPI_Datatype recvtype,
                  MPI_Comm comm)
</PRE>

<PRE>
int MPI_Allgather_c(const void *sendbuf, MPI_Count sendcount,
                    MPI_Datatype sendtype, void *recvbuf, MPI_Count recvcount,
                    MPI_Datatype recvtype, MPI_Comm comm)
</PRE>

<P>
<H2>Input Parameters</H2>
<DL>
<DT><B>sendbuf </B> <DD> starting address of send buffer (choice)

<DT><B>sendcount </B> <DD> number of elements in send buffer (non-negative integer)

<DT><B>sendtype </B> <DD> data type of send buffer elements (handle)

<DT><B>recvcount </B> <DD> number of elements received from any process (non-negative integer)

<DT><B>recvtype </B> <DD> data type of receive buffer elements (handle)

<DT><B>comm </B> <DD> communicator (handle)
</DL>
<P>
<H2>Output Parameters</H2>
<DL>
<DT><B>recvbuf </B> <DD> address of receive buffer (choice)
</DL>
<P>
<H2>Notes</H2>
The MPI standard (1.0 and 1.1) says that
<br>
<P>
<br>
<P>
The jth block of data sent from  each process is received by every process
and placed in the jth block of the buffer <tt>recvbuf</tt>.
<br>
<P>
<br>
<P>
This is misleading; a better description is
<br>
<P>
<br>
<P>
The block of data sent from the jth process is received by every
process and placed in the jth block of the buffer <tt>recvbuf</tt>.
<br>
<P>
<br>
<P>
This text was suggested by Rajeev Thakur and has been adopted as a
clarification by the MPI Forum.
<P>
<H2>Thread and Interrupt Safety</H2>
<P>
This routine is thread-safe.  This means that this routine may be
safely used by multiple threads without the need for any user-provided
thread locks.  However, the routine is not interrupt safe.  Typically,
this is due to the use of memory allocation routines such as <tt>malloc
</tt>or other non-MPICH runtime routines that are themselves not interrupt-safe.
<P>
<H2>Notes for Fortran</H2>
All MPI routines in Fortran (except for <tt>MPI_WTIME</tt> and <tt>MPI_WTICK</tt>) have
an additional argument <tt>ierr</tt> at the end of the argument list.  <tt>ierr
</tt>is an integer and has the same meaning as the return value of the routine
in C.  In Fortran, MPI routines are subroutines, and are invoked with the
<tt>call</tt> statement.
<P>
All MPI objects (e.g., <tt>MPI_Datatype</tt>, <tt>MPI_Comm</tt>) are of type <tt>INTEGER
</tt>in Fortran.
<P>
<H2>Errors</H2>
<P>
All MPI routines (except <tt>MPI_Wtime</tt> and <tt>MPI_Wtick</tt>) return an error value;
C routines as the value of the function and Fortran routines in the last
argument.  Before the value is returned, the current MPI error handler is
called.  By default, this error handler aborts the MPI job.  The error handler
may be changed with <tt>MPI_Comm_set_errhandler</tt> (for communicators),
<tt>MPI_File_set_errhandler</tt> (for files), and <tt>MPI_Win_set_errhandler</tt> (for
RMA windows).  The MPI-1 routine <tt>MPI_Errhandler_set</tt> may be used but
its use is deprecated.  The predefined error handler
<tt>MPI_ERRORS_RETURN</tt> may be used to cause error values to be returned.
Note that MPI does <em>not</em> guarantee that an MPI program can continue past
an error; however, MPI implementations will attempt to continue whenever
possible.
<P>
<DL>
<DT><B>MPI_SUCCESS </B> <DD> No error; MPI routine completed successfully.
</DL>
<DL>
<DT><B>MPI_ERR_BUFFER </B> <DD> Invalid buffer pointer.  Usually a null buffer where
one is not valid.
</DL>
<DL>
<DT><B>MPI_ERR_COMM </B> <DD> Invalid communicator.  A common error is to use a null
communicator in a call (not even allowed in <tt>MPI_Comm_rank</tt>).
</DL>
<DL>
<DT><B>MPI_ERR_COUNT </B> <DD> Invalid count argument.  Count arguments must be 
non-negative; a count of zero is often valid.
</DL>
<DL>
<DT><B>MPI_ERR_TYPE </B> <DD> Invalid datatype argument.  Additionally, this error can
occur if an uncommitted MPI_Datatype (see <tt>MPI_Type_commit</tt>) is used
in a communication call.
</DL>
<DL>
<DT><B>MPI_ERR_OTHER </B> <DD> Other error; use <tt>MPI_Error_string</tt> to get more information
about this error code. 
</DL>
<P>
</BODY></HTML>
